1. Field of the Invention
This invention relates to a coating material for a thermal barrier coating suitable as a starting material for parts of a high temperature exposing apparatus such as a gas turbine, a jet engine or the like as well as a method of producing the same.
2. Background Art
As is well-known, energetic search and development are progressing for the purpose of improving heat efficiency in driving machines such as diesel engine, boiler, gas turbine engine, jet engine and the like. However, the improvement of the heat efficiency results in the strong increase of severe heat loading to the constitutional member. As a metal material used in the high temperature portion of the driving machine, therefore, it is required to have a high mechanical strength under a service environment and excellent resistance to high-temperature oxidation and resistance to high-temperature corrosion.
In order to satisfy such requirements, there are developed many heat-resistant alloys or so-called superalloys consisting essentially of non-ferrous metal elements such as Cr, Ni, Co, W, Ta, Al, Ti or the like.
In these superalloys, however, the high-temperature strength is most preferential, so that it tends to necessarily suppress the addition of the metal element not acting to the improvement of the strength to a low ratio. As a typical example of the metal element not acting to the improvement of the strength are mentioned Cr, Al, Si and the like. On the other hands, these elements are excellent in the oxidation resistance and the resistance to high-temperature corrosion, so that it is common that the oxidation resistance and the resistance to high-temperature corrosion are poor in the superalloy prevailing the high-temperature strength as mentioned above.
In view of the above situation, the lowering of the resistance force to the chemical damage of the superalloy used in the high-temperature environment is compensated by previously coating the surface of the superalloy with a metal of Cr, Al, Si or the like or an alloy thereof through a spraying method or a diffusion penetration method.
Recently, a spraying method of easily forming a coating of an oxide or an alloy having an oxidation resistance and a heat resistance is popularized, and the superiority of the spraying method is further enhanced by the development of spraying material shown as MCrAlX alloy (wherein M is at least one metal selected from Ni, Co and Fe, and X is at least one metal selected from Y, Hf, Ta, Cs, Pt, Ce, Zr, La, Si and Th) developing an excellent oxidation resistance. As a technique relating to the MCrAlX alloy material, there are techniques disclosed in JP-A-59-118847 and JP-A-60-141842.
As a field using the spray coating of the MCrAlX alloy, there is a thermal barrier coating used by working into a high-temperature exposing material (NASA Technical Memorandum: NASA-TM-X3425: hereinafter abbreviated as “TBC” simply). This coating is constructed by forming a coating of MCrAlX alloy as an undercoat and laminating a coating of ZrO2 based ceramic having an excellent heat resistance and a small thermal conductivity as a top coat thereon.
The TBC comprising a combination of MCrAlX alloy coating and the ZrO2 based ceramic coating is utilized as not only a high-temperature exposing material for a gas turbine but also a heat-resistant coating for a centrifugal casting mold (see JP-A-64-87050) or for a transferring roll of molten plate glass (see JP-A-4-260622).
Even in the TBC combining the MCrAlX alloy and the ZrO2 based ceramic, however, there is a problem that only the top coat is frequently peeled off from the interface between both the coatings under a severer environment represented by a recently high-temperature operating environment of the gas turbine to lose the thermal barrier action.
As a countermeasure, there have hitherto been proposed a technique wherein the peeling of the top coat is prevented by forming Al2O3 layer obtained through the oxidation of Al coating layer on the MCrAlX alloy coating as an undercoat to improve the oxidation resistance (e.g. JP-A-62-211387), a method of preventing the peeling of the top coat by adding CaO, SiO2 to ZrO2 of the top coat to generate fine cracks to thereby disperse heat stress (e.g. JP-A-4-36454), a method wherein ZrO2 based ceramic coating formed by the spraying method as a top coat is fused by laser heating and longitudinal cracks are generated in the top coat by utilizing stress in the solidification course to mitigate thermal stress to thereby prevent the peeling of the top coat (e.g. JP-A-58-16094), a method wherein the formation of ZrO2 based ceramic as a top coat is carried out by vapor deposition method using an electron beam heat source and microcrystal of ZrO2 based ceramic is grown into column to mitigate thermal stress (e.g. JP-A-3-87379) and the like.
However, these techniques have attained their objects at a use temperature region (1100-1300° C.) of the gas turbine in the time of filing them, but are insufficient under such a use environment that the recent operating temperature exceeds 1500° C.
Also, the coating generating fine cracks in the top coat or growing ZrO2 based ceramic particles grown into the column indicates the effect of mitigating thermal stress, but comes a problem that corrosive components in the combustion gas (e.g. oxygen, steam, SOx, NOx, NaCl, V2O5, Na2SO4 and the like) penetrate into the inside through the fine crack portion or the columnar crystal to corrode the heat-resistant alloy as an undercoat to thereby lower the joining force of the interface between the top coat and the undercoat.